This invention relates to high molecular weight, highly linear polymers of ethylene and xcex1-olefins, e.g., propylene, which are prepared in the presence of an aluminum-based catalyst system. More specifically, the invention relates to the synthesis of ethylene and xcex1-olefin homopolymers and copolymers in an inert reaction medium in the presence of a catalyst system consisting essentially of an aluminum alkyl compound and a Lewis acid component. An advantage of the present invention is that it enables the synthesis of high molecular weight ethylene and xcex1-olefin polymers without the need for transition metal catalysts, thereby avoiding disposal problems associated with the use of such catalysts.
It is known that the xe2x80x9caufbauxe2x80x9d reaction, in which ethylene is reacted at high temperatures and high pressures to form higher olefins, occurs in two steps. In the first step, ethylene is exposed to a trialkyl aluminum compound at temperatures on the order of 90-120xc2x0 C. and pressures of about 100 psi to form higher aluminum alkyls. In the second step, the temperature is raised to 150xc2x0 C. to displace the higher alkyl groups and to form an xcex1-olefin. While studying this reaction in the early 1950""s, it was discovered that the addition to the reaction mass of earlier transition metal compounds, specifically titanium halides, resulted in the formation of high molecular polymers. Since that discovery, a variety of catalyst systems have been reported. using a variety of transisiton metals, including chromium (IV) oxides (Hogan, J. P., et al, U.S. Pat. No. 2,825,721), chromocenes (Karol, F. J., et al, J. Polym. Sci., Part A, 1972, 2621), and acetylacetonate complexes of vanadium (Doi, Y., et al, Makromol. Chem., 1979, 180, 1359). Beginning in about 1980, a great deal of study was conducted in connection with highly active metallocene/methylaluminoxane (MAO) olefin polymerization catalyst systems, and more recently olefin polymerization catalysts based on diimine complexes of nickel and palladium have been reported. See, e.g., Sinn, H. and Kaminski, W., Adv. Organomet. Chem., 1980, 18, 99; Johnson, L. K., et al, J. Am. Chem. Soc., 1995, 117, 6414; Johnson, L. K., et al, Int. Pat. Appl. W096/23010 (1996); and Small, B. L., et al, J. Am. Chem. Soc., 1998, 120,4049.
For each of the known transition metal-based catalyst systems, it was believed that the transition metal played a vital role in the formation of high molecular weight polymers; and that in the absence of any transition metal, only oligomers would be produced, as in the aufbau reaction. To date, there have been few reports detailing the preparation of high molecular polymers of ethylene via transition metal-free catalyst systems. In 1992, Heinz Martin (a former student of Karl Ziegler) reported the sysnthesis of high molecular weight polyethylene by exposing ethylene to an aluminum alkyl catalyst over a period of several days (Martin, H., Makromol. Chem., 1992, 193, 1283). More recently, the synthesis of cationic aluminum complexes bearing bulky imine type ligands, as well as their potential utility as ethylene polymerization catalysts, has been investigated. See, e.g., Coles, M. P., et al, J. Am. Chem. Soc., 1997, 119, 8125; Coles, M. P., et al. Int. Pat. Appl. W098/40421; Coles, M. P., et al, Organometallics, 1997, 16, 5183; Aielts, S. L., et al, Organometallics, 1998, 17, 3265; Coles, M. P., et al, Organometallics, 1998. 17. 4042; Ihara, E., et al, J. Am. Chem. Soc., 1998, 120, 8277; Bruce, M., et al, J. Chem. Commun., 1998, 2523; Cameron, P. A., et al, Chem. Commun., 1999, 1883; Kim, J. S., et al, J. Am. Chem. Soc., 2000, 122, 5668; and U.S. Pat. No. 5,777,120.
While great strides have been made in the search for new and improved ethylene and xcex1-olefin polymerization catalysts, there remains a need for catalyst systems that are free from transition metals, that comprise only commercially available components, that require no ligand substitution, and that, nonetheless, are capable of efficiently converting monomer to high molecular weight polymer under otherwise conventional polymerization reaction conditions.
In accordance with the present invention, the need for transition metal-free olefin polymerization catalysts has been met by providing a catalyst system that comprises two essential components, namely: (1) an aluminum alkyl component, and (2) a Lewis acid or Lewis acid derivative component that is capable of activating the aluminum alkyl component.
The aluminum alkyl component may be illustrated by the formula AlRxH3xe2x88x92x, where R is an alkyl group, and 0 less than xc3x97xe2x89xa63. Aluminum alkyl compounds that are suitable for use in this invention include, for example, trimethylaluminum, triethylaluminum, triisobutylaluminum, tri-n-octylaluminum and diethylaluminum hydride.
The Lewis acid component contemplated for use in this invention includes Lewis acids and Lewis acid derivatives having a relative Lewis acidity equal to or stronger than that of Tris-(pentafluorophenyl) boron (designated herein as xe2x80x9cFABxe2x80x9d and having the formula B(C6F5)3)), as determined from 1H NMR spectra of the Lewis acid or Lewis acid derivative based on H3 chemical shift changes in crotonaldehyde when the latter is bound to the respective Lewis acid or Lewis acid derivative. The determination of the relative strength of a given Lewis acid or Lewis acid derivative is discussed more fully, for example, in Luo et al, Topics in Catalvsis, 1999, 7, 97, the disclosure of which is incorporated herein by reference. Lewis acid derivatives contemplated for use in this invention are those derivatives that are formed when a hydrocarbyl or halide ion is added to a Lewis acid. For example, when C6F5xe2x88x92 or Clxe2x88x92 is added to B(C6F5)3, the anionic derivatives B(C6F5)4xe2x88x92 and B(C6F5)3(Cl)xe2x88x92, respectively, would be the Lewis acid derivatives.
Thus, the Lewis acid or Lewis acid derivative component, hereinafter sometimes referred to as the xe2x80x9cLewis acid componentxe2x80x9d, contemplated for use in the present invention includes for example, tris-(pentafluorophenyl) boron (designated herein as xe2x80x9cFABxe2x80x9d and having the formula B(C6F5)3)), tri(phenyl)methyl tetra(pentafluorophenyl)borate (designated herein as xe2x80x9cTrityl FABxe2x80x9d and having the formula {(C6F5)3C}+{B(C6F5)4}xe2x88x92), N,N-dimethylanilinium tetra(pentafluorophenyl)borate (designated herein as xe2x80x9cAnilinium FABxe2x80x9d and having the formula {(CH3)2N(H)(C6H5)}+{B(C6F5)4}xe2x88x92), tri(phenyl)methyl tri (pentafluorophenyl)(chloro)borate (having the formula {(C6F5)3C}+{B(C6F5)3(Cl)xe2x88x92}) or a conventional alkylaluminoxane, such as methylaluminoxane (designated herein as xe2x80x9cMAOxe2x80x9d and having the formula xe2x80x94(Al(CH3)O)n)xe2x80x94.
MAO, which is the product of the hydrolysis of trimethylaluminum, contains as much as 30% unreacted aluminum trialkyl. Accordingly, it is within the scope of this invention to use MAO as both the aluminum alkyl component and as the Lewis acid component of the catalyst system. However, in such case, it is preferable to add an aluminum alkyl component and/or a Lewis acid component in addition to the MAO. Similarly, it is also within the scope of this invention to use an alkylaluminoxane in conjunction with an alcohol or phenol adduct of an alkylaluminoxane. For example, a suitable catalyst system in accordance with this invention would comprise MAO in combination with 2,6-di-t-butylphenol.MAO.
The catalyst system of this invention is indeed capable of polymerizing ethylene and xcex1-olefins, particularly propylene, under conventional reaction conditions, and results in the formation of high molecular weight, highly linear polymers having narrow polydispersities, indicative of a xe2x80x9csingle sitexe2x80x9d catalyst.
The polymerization typically is carried out by contacting the selected monomer (e.g., ethylene and/or propylene) in an inert polar solvent (e.g., chlorobenzene) or hydrocarbon solvent (e.g., toluene) at a temperature of about 20 to 150xc2x0 C., typically from about 50 to about 120xc2x0 C., e.g. 50xc2x0 C, and a pressure of from about 50 to about 1,500 psi, typically from about 400 to about 800 psi, e.g., 800 psi. The polymerization reaction typically would be allowed to proceed for a period of from about 1 hour to about 24 hours, after which the polymerization reaction would be terminated by conventional means, e.g., by adding methanol or another conventional polymerization stopper to the reaction mass.